1. Field of the Invention
The present invention relates to a semiconductor thin film formed on a substrate having an insulating surface and a semiconductor device using such a semiconductor thin film as its active layer. In particular, the invention relates to such a semiconductor thin film and semiconductor device in which the semiconductor thin film is made of a material having silicon as the main component.
Further, the invention relates to a semiconductor circuit and an electro-optical device that are constituted of semiconductor devices such as thin-film transistors as well as to an electronic apparatus using such a semiconductor circuit and electro-optical device.
In this specification, the term “semiconductor device” is used as including all of the above-mentioned thin-film transistor, semiconductor circuit, electro-optical device, and electronic apparatus; that is, all of devices and apparatuses that function by utilizing a semiconductor characteristic are called semiconductor devices. Therefore, the semiconductor devices recited in the claims are not limited to single elements such as a thin-film transistor and encompass semiconductor circuits and electro-optical devices that are constructed by integrating such single elements as well as electronic apparatuses using such a semiconductor circuit or electro-optical device as a part.
2. Description of the Related Art
In recent years, the techniques of forming thin-film transistors (TFTs) by using a semiconductor thin film (thickness: tens to hundreds of nanometers) formed on a substrate having an insulating surface have attracted much attention. Thin-film transistors particularly as switching elements of image display devices such as liquid crystal display devices are now being developed at high speed.
For example, in liquid crystal display devices, it is attempted to apply TFTs to every kind of electric circuit such as a pixel matrix circuit in which pixel regions arranged in matrix form are controlled individually, a driver circuit for controlling a pixel matrix circuit, or a logic circuit (an operation circuit, a memory circuit, a clock generator, or the like) for processing an external data signal.
At present, TFTs using an amorphous silicon film as an active layer have been put into practical use. However, TFTs using a crystalline silicon film such as a polysilicon film are necessary for electric circuits, such as a driver circuit and a logic circuit, that are required to operate at even higher speed.
For example, techniques of the present assignee that are disclosed in Japanese Laid-open Patent Publication Nos. Hei. 7-130652 and Hei. 8-78329 are known as methods for forming a crystalline silicon film on a glass substrate. The disclosures of which are incorporated herein by reference. By utilizing a catalyst element for accelerating crystallization of an amorphous silicon film, the techniques of these publications enable formation of a crystalline silicon film having superior crystallinity by a heat treatment of 500°-600° and about 4 hours.
In particular, the technique of the publication No. 8-78329 is such that the above technique is utilized to cause crystal growth in a direction approximately parallel with the substrate surface. The present inventors especially call a resulting crystallized region a “lateral growth region.”
However, even a driver circuit that is constructed by using such TFTs cannot completely provide required performance. In particular, at present, it is impossible to construct, by using conventional TFTs, high-speed logic circuits that are required to operate at extremely high speed (megahertz to gigahertz).
To improve the crystallinity of a crystalline silicon film having grain boundaries (called a polysilicon film), the inventors have repeated trial and error as exemplified by a semi-amorphous semiconductor (Japanese Laid-open Patent Publication No. Sho. 57-160121 etc.) and a monodomain semiconductor (Japanese Laid-open Patent Publication No. Hei. 8-139019). The disclosures of which are incorporated herein by reference.